Rotary piston vacuum pump with washing installation

ABSTRACT

A pump having a rotor, a stator, a housing enclosing the rotor and the stator, the housing having an inlet for a fluid, and a port for receiving a fluid which acts on deposits on a surface of the rotor and on a surface of the stator positioned downstream from the inlet.

FIELD OF THE INVENTION

This invention relates to the field of vacuum pumps. In particular, butnot strictly limited to vacuum pumps with a screw type configuration.

BACKGROUND OF THE INVENTION

Screw pumps usually comprise two spaced parallel shafts each carryingexternally threaded rotors, the shafts being mounted in a pump housingsuch that the threads of the rotors intermesh. Close tolerances betweenthe rotor threads at the points of intermeshing and with the internalsurface of the pump body, which typically acts as a stator, causesvolumes of gas being pumped between an inlet and an outlet to be trappedbetween the threads of the rotors and the internal surface and therebyurged through the pump as the rotors rotate.

Screw pumps are widely regarded as a reliable means for generatingvacuum conditions in a multitude of processes. Consequently, they arebeing applied to an increasing number of industrial processes. Suchapplications may involve materials that have “waxy” or “fatty”properties e.g. tallow based plasticisers. In operation of the pump,these products form deposits on the surfaces of the pump. On shutdown ofthe pump these surfaces cool, the deposits also cool and solidify withinthe pump. Where such deposits are located in clearance regions betweencomponents, they can cause the pump to seize up such that restart isinhibited or even prevented.

Similar problems can be encountered in a number of semiconductorprocesses that use vacuum pumps, especially those in the chemical vapourdeposition (CVD) category 200 shown for example in FIG. 10. Suchprocesses can produce a significant amount of by-product material 202.This can be in the form of powder or dust, which may remain loose orbecome compacted, or in the form of hard solids, especially if theprocess gas is condensable and sublimes on lower temperature surfaces.This material can be formed in the process chamber 201, in the foreline204 between the chamber and the pump, and/or in the vacuum pump 203itself. If such material accumulates on the internal surfaces of thepump during its operation, this can effectively fill the vacant runningclearance between the rotor and stator elements on the pump, and canalso cause spikes in the current demand on the motor of the vacuum pump.If this continues unabated, then this build-up of solid material caneventually cause the motor to become overloaded, and thus cause thecontrol system to shut down the vacuum pump. Should the pump be allowedto cool down to ambient temperature, then this accumulated material willbecome compressed between the rotor and stator elements. Due to therelatively large surface area of potential contact that this createsbetween the rotor and stator elements, such compression of by-productmaterial can increase the frictional forces opposing rotation by anorder of magnitude.

In order to release the rotors in prior art pumps, a facility isprovided whereby a bar can be inserted into sockets attached to theprimary shaft of the rotor though an access panel. This bar is used as alever to try to rotate the shaft and release the mechanism such that themachine can be restarted. This levering system allows more rotationalforce to be applied to the internal components than could be exerted bythe motor. Such force will be transmitted to the rotor vanes and theassociated stresses may prove to be detrimental to the structure of therotor. If this system fails to release the mechanism it is thennecessary to disassemble the apparatus such that a liquid solvent can bepoured into the pump casing to dissolve the residue to a level where theshaft can be rotated manually. This disassembly not only causes the pumpto be off line for a certain length of time, but it then must bere-commissioned and re-tested to ensure the reliability of theconnections to the surrounding apparatus.

It is an aim of the present invention to overcome the aforementionedproblems associated with pump technology.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a pump comprising a rotor element and astator element; a housing enclosing the elements and having an inlet forreceiving pumped fluid, and downstream from the inlet, at least oneport; and means for injecting, into the housing via said at least oneport, fluid for acting on deposits located on the element surfaces toenable said deposits to be removed therefrom. As the port(s) are locateddownstream of the inlet, any fluid injected on the rotor and statorelements can be directly injected into the swept volume to impinge onthe surfaces of these elements. This can significantly improve cleaningefficiency in comparison to a system where the cleaning fluid isintroduced via the housing inlet for pumped fluids. Where many ports areprovided, these may be located in an array. For example, the ports maybe located radially about the housing, and/or may be located along thelength of the rotor element.

The housing may comprise an inner layer and an outer layer between whicha cavity may be formed. In operation of the pump a liquid may be passedthrough this cavity. The inner layer of the housing may act as thestator of the pump.

The port may include a nozzle through which, in use, fluid is sprayed,this nozzle may be integrally formed within the port.

The pump may be a screw pump 30 a comprising two threaded rotors inwhich case the port(s) may be located after the first two complete turnsof thread of the rotors from the inlet end of the rotor. Alternativelythe pump may be a Northey (“claw”) pump 30 b or a Roots pump 30 c asshown in FIG. 5 to include an arrangement for supplying fluid to a pumpin accordance with the present invention.

The fluid may be a liquid or a vapour. The fluid may be a solvent fordissolving residue collected on the rotor when the pump is in use or itmay be steam. The fluid may comprise a reactive substance for reactingwith the deposits, and may comprise, for example, a halogen. Such fluidcan be particularly useful as a cleaning fluid when the pump is used aspart of a CVD process to remove solid by-products of the CVD process.

Thus, the present invention also provides a pump comprising a rotorelement and a stator element; a housing enclosing the elements andhaving at least one port; and means for injecting, into the housing viasaid at least one port, a fluid comprising a reactive substance forreacting with particulates located on the element surfaces to enablesaid particulates to be removed therefrom.

The fluid may comprise a halogen, for example fluorine, and may be afluorinated gas, such as a perfluorinated gas. Examples of such fluidinclude ClF₃, F₂, and NF₃.

The invention thus extends to chemical vapour deposition apparatus 32comprising a process chamber 31 and a pump according to any precedingclaim for evacuating the process chamber, wherein, in use, the depositsare a by-product of a chemical vapour deposition process.

According to the present invention there is further provided a method ofmanaging deposits within a pump, the pump comprising a rotor element anda stator element, and a housing enclosing the elements and having aninlet for receiving pumped fluid, and downstream from the inlet, atleast one port, the method comprising injecting, into the housing viasaid at least one port, fluid for acting on deposits located on theelement surfaces to enable said deposits to be removed therefrom.

The present invention also provides a method for managing depositswithin a pump, the pump comprising a rotor element and a stator element,and a housing enclosing the elements and having at least one port; themethod comprising injecting, into the housing via said at least oneport, a fluid comprising a reactive substance for reacting withparticulates located on the element surfaces to enable said particulatesto be removed therefrom.

Referring to FIG. 6, the delivery of fluid may occur at predeterminedintervals during operation of the pump, for example, using solenoidvalve control. Furthermore a monitoring step 100 may be performedwherein the performance of the pump is monitored, for example, bymeasuring at least one of the group of rotor speed, power consumption,and volumetric gas flow rate. These measured parameters may be used todetermine the extent of accumulation of deposits on the internal workingsurfaces of the pump 101. A fluid flow rate may then be calculated, thisrate being that of the delivered fluid that would be sufficient tocompensate for the quantity of accumulated deposits 102 as determinedabove. Subsequently, the flow rate of fluid being delivered to the rotormay be adjusted 103 to reflect the new calculated value.

Referring to FIG. 7, according to the present invention there is furtherprovided a method for managing deposits within a pump mechanism byintroducing fluid suitable for dissolving, diluting or otherwisedisengaging deposits which have accumulated on the internal workingsurfaces of the pump, the method comprising the steps of:

-   -   (a) monitoring the performance of the pump 110, for example, by        recording at least one of the group of rotor speed, power        consumption, and volumetric gas flow rate;    -   (b) calculating the rate of accumulation of deposits on the        internal working surfaces of the pump based on the monitored        performance 111;    -   (c) calculating a fluid flow rate required to compensate for the        accumulation of deposits as determined in step (b) 112; and    -   (d) effecting an adjustment of the flow rate of fluid being        delivered to the rotor to reflect the calculated value from        step (c) 113.

The pump may be inoperative as the fluid is delivered, for example whereseizure has occurred or where cleaning needs to take place. Referring toFIG. 8, in this case, the method may further involve applying torque 114to the rotors of the pump in order to overcome any remaining impedingforce potentially caused by deposits located on the internal workingcomponents of the pump. Under certain conditions, for example where thematerial being transported is particularly viscous or waxy and thisviscosity may reduce with an increase in temperature, the method mayfurther involve the introduction of thermal fluid 115 into a cavityprovided within the housing of the pump, where this cavity encircles therotor components. This thermal fluid may be heated 116 in order to raisethe temperature of the fluid and the deposits sufficiently to releasethe deposits prior to applying the torque as discussed above. (FIG. 9).

The controller of the dry pump apparatus may comprise a microprocessorwhich may be embodied in a computer, which in turn is optionallyprogrammed by computer software which, when installed on the computer,causes it to perform the method steps (a) to (d) mentioned above. Thecarrier medium of this program may be selected from but is not strictlylimited to a floppy disk, a CD, a mini-disc or digital tape.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the present invention will now be described with referenceto the accompanying drawings in which:

FIG. 1 illustrates a schematic of a screw pump of the present invention;

FIG. 2 illustrates a schematic of a double-ended screw pump of thepresent invention;

FIG. 3 is an end sectional view of the pump of FIGS. 1 and 2;

FIG. 4 is a detailed view of a section of a water jacket thatillustrates the implementation of an injection port; and

FIG. 5 illustrates an arrangement for supplying fluid to a pump

FIGS. 6-9 illustrate flowcharts in accordance with embodiments of thepresent invention.

FIG. 10 illustrates a schematic view of a conventional chemical vapordeposition system.

DETAILED DESCRIPTION

Whilst the example pumps illustrated in FIGS. 1 and 2 are screw pumps itis envisaged that this invention can be applied to any type of vacuumpump, in particular claw pumps.

In the example of FIG. 1, two rotors 1 are provided within an outerhousing/stator 5 where the outer housing serves as the stator of thepump. The two contra-rotating, intermeshing rotors 1 are positioned suchthat their central axes lie parallel to one another. The rotors aremounted through bearings 10 and driven by a motor 11 (shown in FIG. 2).Injection ports 2 are provided along the length of the rotor, in theexamples of FIGS. 1 and 2 (shown as solid lines in FIG. 3) these ports 2are located laterally within the pump on the opposite side of the rotorsfrom the intermeshing region of the rotors. However, the ports may bepositioned at any radial location around the outer housing/stator 5.Some of these locations are illustrated in FIG. 3.

The ports 2, which may contain nozzles 2 a to allow the fluid to besprayed, are preferably distributed along the length of the outerhousing/stator 5 such that the solvent or steam can be easily appliedover the entire rotor. Alternatively, this distribution of ports allowsthe fluid to be readily concentrated in any particular problem area thatmay arise. This is especially important when solvent is injected duringoperation, in order to limit the impact on pump performance. If, forexample, a single port was to be used at the inlet 3 of the pump, thismay have a detrimental effect on the capacity of by-products that couldbe transported away from the evacuated chamber (not shown) by the pump.By bringing solvent into contact with the rotor 1 after the first fewturns of the thread, the likelihood of backward contamination of thesolvent into the chamber will be reduced.

Furthermore, where solvent is introduced in the inlet region of thepump, the pressure is such at the inlet that there is an increased riskthat the solvent will flash. In processes where it is necessary for thesolvent to remain in liquid phase the solvent must be introduced closertowards the exhaust region of the pump where the pressures will haverisen. As solvent is introduced through a number of ports 2 along thelength of the outer housing/stator 5, the overall effect is to graduallyincrease the quantity of solvent present, as the likelihood of residuebuild up on the rotor 1 increases towards the exhaust stages. Anadditional benefit may be seen in some configurations where addition ofliquid into the final turns of thread of the rotor will act to seal theclearances between the rotor and the stator in this region of the pump.Thus leakage of gas will be substantially reduced and performance of thepump will be improved.

In some processes, it is not appropriate to introduce solvent duringoperation as the waste products from the evacuated chamber are collectedat the outlet of the pump for a particular purpose and this materialought not to be contaminated. Other applications may not result inlevels of residue that warrant constant injection of solvent duringoperation. In these cases, and where an unplanned shut down of the pumpoccurs such that standard practices, such as purging, are not followed,the residue from the process cools down as the apparatus drops intemperature. In these circumstances a seizure of the mechanism may occuras deposits build up and become more viscous or solidify. In a systemaccording to the present invention, the injection ports 2 can be used tointroduce a solvent into the stator cavity 6 in a distributed mannerwithout needing to go to the expense or inconvenience of disassemblingthe apparatus. Once the solvent has acted upon the deposits to eithersoften or dissolve them, the shaft may then be rotated either by usingthe motor or manually to release the components without applyingexcessive, potentially damaging, force to the rotor.

Delivery of fluid may be performed through simple ports as liquid isdrip-fed through a hole in the housing or nozzles 2 a may be providedthrough which the fluid may be sprayed. Control systems may beintroduced such that the solvent delivery can be performed in reactionto the changing conditions being experienced within the confines of thepump apparatus. For example, in the arrangement shown in FIG. 5, acontrol system 20 supplies cleaning fluid, for example, stage by stage,to the ports 2 of pump 21 via supply conduits 22. As indicated at 24, apurge gas system may also be provided for supplying a purge gas, such asnitrogen to the pump 21.

Where the process material is waxy or fatty, compatible solvents willneed to be introduced to perform the dilution/cleaning function. Suchsolvents may be provided in liquid or vapour form. Any compatible,effective cleaning medium may be used such as xylene in the case ofhydrocarbon based/soluble products or water in the case of aqueousbased/soluble products, alternatively, detergents may be used.

Where the process material is a by-product of a CVD process, thecleaning fluid may comprise a fluorinated gas. Examples of such cleaningfluid include, but are not restricted to, ClF₃, F₂, and NF₃. The highreactivity of fluorine means that such gases would react with the solidby-products on the pump mechanism, in order to allow the by-products tobe subsequently flushed from the pump with the exhausted gases. To avoidcorrosion of internal components of the pump by the fluorinated gases,materials need to be carefully selected for use in forming components ofthe pump, such as the rotor and stator elements, and any elastomericseals, which would come into contact with the cleaning gas.

The outer housing/stator 5 as illustrated in FIG. 3 is provided as atwo-layer skin construction, an inner layer 6 a and an outer layer 9. Itis the inner layer 6 a that acts to define the stator cavity 6 of thepump. A cavity 7 is provided between the layers 6 a and 9 of the outerhousing/stator 5 such that a cooling fluid, such as water, can becirculated around the stator in order to conduct heat away from theworking section of the pump. This cavity 7 is provided over the entirelength of the rotor i.e. over the inlet region 3 as well as the exhaustregion 4. Under circumstances where the pump has become seized due tocooling of the rotor which, in turn, solidifies residues on the surfacesbetween the rotor and the stator, the ‘cooling liquid’ in the cavity 7of the outer housing/stator 5 may be heated to raise the temperature ofthe rotor 1. This can enhance the pliability of the residue and mayassist in releasing the mechanism. The outer housing/stator 5 isprovided with pillars 8 of solid material through the cavity 7 in orderto provide regions where injection ports 2 can be formed.

The present invention is not restricted for use in screw pumps and mayreadily be applied to other types of pump such as Northey (“claw”) pumpsor Roots pumps.

In summary, a pump comprises at least one rotor 1, a stator/outerhousing 5, the rotor 1 being enclosed by the outer housing/stator 5. Theouter housing/stator 5 comprises at least one port 2 extending throughthe outer housing/stator 5 to enable delivery of a fluid directly onto asurface of the at least one rotor 1.

It is to be understood that the foregoing represents just a fewembodiments of the invention, others of which will no doubt occur to theskilled addressee without departing from the true scope of the inventionas defined by the claims appended hereto.

1. A pump comprising: a rotor and a stator; a housing enclosing therotor and the stator, the housing having an inlet for receiving a firstfluid, and a port positioned downstream and spaced apart from the inlet;and means for injecting a second fluid into the housing through the portin a first direction not in direct opposite to a second direction inwhich the first fluid flows into the housing via the inlet, wherein thesecond fluid acts on deposits on a surface of the rotor and a surface ofthe stator, and wherein the second fluid comprises a reactive substancefor reacting with deposits on the surface of the rotor and the surfaceof the stator.
 2. The pump according to claim 1 comprising a pluralityof ports.
 3. The pump according to claim 2 wherein the ports are locatedradially about the housing.
 4. The pump according to claim 2 wherein theports are located along a length of the rotor.
 5. The pump according toclaim 4 wherein at least one of the ports includes a nozzle for sprayingthe second fluid.
 6. The pump according to claim 5 wherein the nozzle isintegrally formed within at least one of the ports.
 7. The pumpaccording to claim 2 wherein at least one of the ports includes a nozzlefor spraying fluid.
 8. The pump according to claim 7 wherein the nozzleis integrally formed within at least one of the ports.
 9. The pumpaccording to claim 8 wherein the housing comprises a two skinned wallhaving an inner skin and an outer skin and forming a cavity between theinner and outer skins.
 10. The pump according to claim 9 wherein theinner skin of the housing is adapted to form the stator.
 11. The pumpaccording to claim 7 wherein the second fluid is a liquid.
 12. The pumpaccording to claim 11 wherein the second fluid is a solvent.
 13. Thepump according to claim 7 wherein the second fluid is a gas.
 14. Thepump according to claim 13 wherein the second fluid is steam.
 15. Thepump according to claim 7 wherein the second fluid comprises a reactivesubstance for reacting with the deposits.
 16. The pump according toclaim 15 wherein the second fluid comprises a halogen.
 17. The pumpaccording to any of claim 15 wherein the second fluid comprises acompound selected from the group consisting of ClF₃, F₂, and NF₃. 18.The pump according to claim 1 wherein the pump is a screw pump havingtwo threaded rotors.
 19. The screw pump according to claim 18 whereinthe port is located downstream of a first two complete turns of threadof the threaded rotors.
 20. The pump according to claim 1 wherein thepump is a claw pump.
 21. The pump according to claim 1 wherein the pumpis a Roots pump.
 22. The pump according to claim 1 wherein the secondfluid is a liquid.
 23. The pump according to claim 1 wherein the secondfluid is a solvent.
 24. The pump according to claim 1 wherein the secondfluid is a gas.
 25. The pump according to claim 24 wherein the secondfluid is steam.
 26. The pump according to claim 1 wherein the housingcomprises a two skinned wall having an inner skin and an outer skin andforming a cavity between the inner and outer skins.
 27. The pumpaccording to claim 26 wherein the inner skin of the housing is adaptedto form the stator.
 28. The pump according to claim 1 wherein the pumpis connected to a chemical vapor deposition apparatus having a processchamber and an outlet of the process chamber, wherein the pump inlet isconnected to the outlet of the process chamber, and wherein the depositsare a by-product of a chemical vapor deposition process.
 29. A pumpcomprising: a rotor and a stator; a housing enclosing the rotor and thestator and having an inlet for receiving a first fluid, and a portpositioned downstream and spaced apart from the inlet; and means forinjecting a fluid into the housing through the port in a first directionnot in direct opposite to a second direction in which the first fluidflows into the housing via the inlet, wherein the fluid comprises areactive substance for reacting with particulates on a surface of therotor and a surface of the stator.
 30. The pump according to claim 29wherein the fluid comprises a halogen.
 31. The pump according to claim29 wherein the fluid comprises a compound selected from the groupconsisting of ClF₃, F₂, and NF₃.
 32. A method of managing depositswithin a pump, the pump comprising a rotor and a stator, and a housingenclosing the rotor and the stator, the housing having an inlet forreceiving a first fluid, and downstream, spaced apart from the inlet, aport, the method comprising: injecting into the housing via the port asecond fluid for acting on deposits on a surface of the rotor and asurface of the stator, wherein the second fluid is injected into thehousing in a first direction not in direct opposite to a seconddirection in which the first fluid flows into the housing via the inlet,and wherein the second fluid comprises a reactive substance for reactingwith the deposits on the surface of the rotor and the surface of thestator.
 33. The method according to claim 32 wherein the second fluid isinjected from a plurality of ports.
 34. The method according to claim 33wherein the ports are located radially about the housing.
 35. The methodaccording to claim 33 wherein the ports are located along a length ofthe rotor.
 36. The method according to claim 33 wherein the second fluidis injected through the ports at predetermined intervals.
 37. The methodaccording to claim 32 wherein the second fluid is a liquid.
 38. Themethod according to claim 32 wherein the second fluid is a solvent. 39.The method according to claim 32 wherein the second fluid is a gas. 40.The method according to claim 39 wherein the second fluid is steam. 41.The method according to claim 32 wherein the second fluid comprises areactive substance for reacting with the deposits.
 42. The methodaccording to claim 32 wherein the second fluid comprises a halogen. 43.The method according to claim 32 wherein the second fluid comprises acompound selected from the group consisting of ClF₃, F₂, and NF₃. 44.The method according to claim 32 wherein the second fluid is injectedthrough the port at predetermined time intervals.
 45. The methodaccording to claim 32 further comprising the steps of: (a) monitoringthe performance of the pump; (b) determining accumulation of thedeposits on the internal surfaces based on the monitored performance;(c) calculating a rate of flow of the second fluid required tocompensate for the accumulation of the deposits; and (d) adjusting therate of flow of the second fluid to reflect the calculated rate of flowof the second fluid.
 46. A method for managing deposits within a pumpmechanism by delivering to a rotor of the pump, a fluid for dissolving,diluting or otherwise disengaging deposits which have accumulated on theinternal working surfaces of the pump, the method comprising the stepsof: (a) monitoring the performance of the pump; (b) calculating the rateof accumulation of the deposits on the internal working surfaces of thepump based on the monitored performance; (c) calculating a rate of flowof the fluid, required to compensate for the accumulation of thedeposits; (d) adjusting the rate of flow of the fluid being delivered tothe rotor to reflect the calculated rate of flow of the fluid; whereinthe pump is inoperative as the fluid is delivered, the method furthercomprising the step of applying torque to rotors of the pump to overcomeany remaining impeding force.
 47. A method for managing deposits withina pump mechanism by delivering to a rotor of the pump, a fluid fordissolving, diluting or otherwise disengaging deposits which haveaccumulated on the internal working surfaces of the pump, the methodcomprising the steps of: (a) monitoring the performance of the pump; (b)calculating the rate of accumulation of the deposits on the internalworking surfaces of the pump based on the monitored performance; (c)calculating a rate of flow of the fluid, required to compensate for theaccumulation of the deposits; (d) adjusting the rate of flow of thefluid being delivered to the rotor to reflect the calculated rate offlow of the fluid; wherein the pump is inoperative as the fluid isdelivered; the method further comprising the steps: applying torque torotors of the pump to overcome any remaining impeding force; introducinga thermal fluid into a cavity formed within a housing of the pump, thecavity encircling the rotors; and heating the thermal fluid in thecavity to raise the temperature of the fluid and the deposits to releasethe deposits prior to the step of applying torque to the rotors.